Objective: The objective of the present study was to assess the prevalence and severity of symptoms of posttraumatic stress disorder (PTSD), as well as its risk factors, among children and adolescents under 18 who had been exposed to earthquake. Method: This cross-sectional study was carried out on 48 children and adolescents under 18 who had been referred to the MDM clinic between the 3rd and the 4th weeks after the earthquake. The tools used in this study included the Child Posttraumatic Symptom Scale (CPSS) and the Screening Tool for Early Predictors of PTSD (STEPP) questionnaires. Results: The prevalence of posttraumatic stress disorder for the age groups below seven, seven to 12, and 13 to 18, was 85%, 74.1%, and 84.6% respectively. In the 13 to 18 age group, the frequency of each category of symptoms was less than in the seven to 12 year group. In children and adolescents above seven years of age, lack of knowledge for parents’ whereabouts, fear of dying and parents’ helplessness had significant association with the diagnosis of PTSD. Conclusion: The prevalence and symptom severity of PTSD is different for different age groups of children and adolescents. Therefore, more accurate tools are probably needed for the diagnosis of this disorder among children and adolescents.